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User blog:Samuli.seppanen/More shots, more lessons learned
The cheiroballistra project is going full steam ahead again. Hopefully now that all my studies are finished I can finally finish this pet project in a reasonable timeframe. Anyways, earlier this week I armed the torsion springs using ~250 hz pretensioning - the same amount I used earlier. This time the springs are housed in the new field-frames. Once the springs were armed, I had to modify the elm arms a bit to prevent the hoops from cutting the spring cord: Small pieces of thick leather bound to the protruding parts of the hoops seemed to prevent wear of the cords adequately. I shot total of 65 shots with the cheiroballistra, gradually increasing the draw length and washer rotation and experimenting with two bowstrings of different lengths (66 cm and 69 cm). Ignoring the details, the best result with 5 shot average the 30 gram bolt was 57.60 m/s, which gives 48.58 joules of energy. This is clearly not adequate: * It roughly equals the energy output of a 60 pound handbow, which in itself is fairly light for war. * It's trivial to make a crossbow/arcuballista that exceeds this performance, especially if using a composite bow. * Cheiroballistra weights about 9 kilos and is a very complex weapon that was probably fairly expensive to manufacture. I would be somewhat happy with twice the amount of output energy (~100 J) when using light bolts (<20 grams). I'd like to be able to eventually reach 150 joule levels. Even a fairly modest energy output can perhaps be forgiven if the cheiroballistra can kill (unarmored) enemies at, say, 400 meters or more. Fortunately there are still many, many things can be tuned to increase performance. I split these into three categories: # Increasing energy storage # Decreasing energy wastage # Improving cocking technique (a.k.a. increasing energy storage potential) There is probably still a lot of untapped potential in the springs. I'll increase energy storage in the springs by increasing washer rotation and by optimizing the form of the force-draw curve with minor modifications to field-frame spacing, bowstring length and initial arm angle. Plenty of energy is still wasted by the overly heavy 93 gram arms. Fortunately I have two extra pairs of lighter hooks ready: using these hooks the (new) arms should weigh around 70 grams and 63 grams respectively Also, the nylon bowstrings I've used are probably way too heavy for the cheiroballistra (10-13 grams). I estimate that at least half of that weight can be removed safely because the torsion springs - not the bowstring - will arrest the movement of the arms. This of course only happens if the bowstring is either very elastic or very slack. Having a slack bowstring is a very bad idea in bows, and almost certainly in ballistas also, so I've chosen to make elastic ones from nylon. In the final version the bowstring will be made from "best quality sinews" as suggested one of the ancient authors. At the final phase of the shot the bowstring will be stretched by the residual energy in the arms until the hoops of the arms hit the curves in the field-frames. From this point onward the curve will act as a pivot point and the heel of the arm will force one half of the torsion spring outwards. This should consume any the residual energy in the springs safely. Based on my still somewhat limited experience this is what actually happens. Third thing that can be improved is improve the cocking technique. Currently I've had to use one hand to manage a loose 12 mm steel pin that locks the slider via the handle to the stock. This means that really aggressive pushes as well assisting the pull with hands has been ruled out. To solve this issue I intend to modify the pin so that the handle automatically locks to it without any help from the operator. This modification will not require any extra parts, only minor modification of the pin. Category:Blog posts Category:Theoretical Category:Backup Category:Statistics Category:Cheiroballistra